1. Field of the Invention
The present invention relates generally to integrated circuits for driving an active or passive matrix liquid crystal display (LCD) or the like, and more particularly, to an integrated circuit which provides a relatively high voltage output driver signal while retaining smaller device geometries allowed by using low voltage CMOS processing.
2. Description of the Background Art
Active matrix LCD displays are used today in a variety of products, including hand-held games, hand-held computers, and laptop/notebook computers. These displays are available in both gray-scale and color forms, and are typically arranged as a matrix of intersecting rows and columns. The intersection of each row and column forms a pixel, or dot, the density and/or color of which can be varied in accordance with the voltage applied thereto in order to define the gray shades of the liquid crystal display. These various voltages produce the different shades of color on the display, and are normally referred to as "shades of gray" even when speaking of a color display.
It is known to control the image displayed on the screen by individually selecting one row of the display at a time, and applying control voltages to each column of the selected row. This process is carried out for each individual row of the screen. After each row has been selected, the process is repeated to refresh and/or update the displayed image.
LCD displays used in computer screens require a large number of column driver outputs which must operate at voltages from 8 volts to as high as 20 volts (hereinafter, "high voltage"). Color displays usually require three columns per pixel, one for each of the three primary colors to be displayed. Thus, a typical VGA (480 rows.times.640 columns) color liquid crystal display includes 640.times.3, or 1,920 column lines which must be driven at high voltage by a like number of column driver outputs.
The column driver circuitry is typically formed upon monolithic integrated circuits. Assuming that an integrated circuit can be provided with 192 column output drivers, then a color VGA display screen requires 10 of such integrated circuits (10.times.192=1,920). Due to the relatively large number of such column driver integrated circuits that are required by such a color VGA display screen, the cost of such column driver integrated circuits can greatly influence the overall cost of the display.
CMOS is the most widely used technology for integrated circuits today. However, the magnitude of the voltage that can be used to power CMOS circuits is dependent upon the physical dimensions of the individual transistors and the particular processing utilized to manufacture the transistors. In general, as the required operating voltage is increased, the individual transistors must be made larger, and the processing of the integrated circuits becomes more complex. This equates to additional cost for the device since the total chip area of the integrated circuit and the complexity of the manufacturing process are major factors in the cost of an integrated circuit. Increased transistor area means lower device density, larger integrated circuits, lower yields, and higher cost per integrated circuit. In order to increase the voltage tolerance of CMOS transistors to breakdown conditions, the dimensions of the CMOS transistors must be increased in a linear relationship to the amount of voltage tolerance desired. Thus, if the CMOS transistors must have double the breakdown voltage tolerance, then the dimensions of such transistors must be roughly doubled. However, overall chip area goes up by the square of transistor dimensions, so doubling device spacings can result in an increase in required chip area by a factor of four.
In addition, the reliability of a CMOS integrated circuit is directly affected by the voltage at which it is operated and by the sizes of the individual transistors. This is due to two different phenomena observed in CMOS integrated circuits. One such phenomena is a breakdown mechanism caused by a larger electric field imposed across the gate oxide of the CMOS transistors. The voltage at which the gate oxide of a CMOS transistor may break down in actuality is typically less than the theoretical breakdown voltage that might be predicted for a given oxide thickness. Defects in the gate oxide can allow the gate oxide of the CMOS device to break down over time at voltages far below the theoretical breakdown voltage for such given oxide thickness. For example, the theoretical gate oxide breakdown voltage for a 1.2 micron CMOS process might be 18 volts; in practice, however, the voltage across such gate oxide must be maintained below 8 volts or the reliability of the integrated circuit will be compromised over time.
A second phenomena observed in CMOS circuits is a problem related to channel breakdown wherein the application of voltages that exceed a certain maximum voltage limit across the source and drain terminals of CMOS transistors can, over time, compromise the reliability of the devices, even though such source-to-drain voltages may be less than the predicted theoretical source-to-drain channel breakdown voltage value. Additional voltage sensitive mechanisms include junction breakdown and field threshold effects which can impair performance of an integrated circuit. These considerations ordinarily dictate the use of larger geometry, more complex processes for high voltage CMOS circuits. Thus, as used in this specification and the accompanying claims, the term "breakdown voltage" should be understood to refer to the maximum practical voltage, or safe operating voltage, which may be applied across the gate oxide of the CMOS transistors and/or across the source and drain terminals of such CMOS transistors to ensure adequate reliability, rather than the actual or theoretical voltages at which such breakdown is observed.
As indicated above, integrated circuits which serve as column drivers for active matrix LCD displays must provide output voltages in the range of 8 to 20 volts. This is a relatively high voltage for CMOS circuits, and the higher operating voltage generally requires that the area of each CMOS switching transistor be larger due to the need for larger geometry processes with thicker gate oxides (to reduce the electric field) and longer channels in order to tolerate the higher operating voltage. As explained above, for a particular CMOS process, there is a maximum voltage which can be applied across each CMOS transistor in order to ensure adequate reliability; this maximum voltage can be increased by increasing the size of each CMOS transistor and thickening the gate oxide insulating layer, but such CMOS processing is more expensive. Hence, the high output voltages required to drive LCD displays has made the column driver integrated circuits relatively expensive. Moreover, because a large number of column driver integrated circuits are needed to drive a display, the column driver integrated circuits represent a significant portion of the total cost of the display.
Accordingly, it is an object of the present invention to generate the higher voltage output signals required to drive the columns of an active or passive matrix LCD display while still being able to retain the smaller device geometries of lower-voltage CMOS integrated circuits produced using lower-cost low-voltage CMOS processing.
It is another object of the present invention to reduce the cost of column driver integrated circuits used to drive active or passive matrix LCD displays.
It is still another object of the present invention to provide column driver integrated circuits used to drive active or passive matrix LCD displays which integrated circuits, when compared with known column driver integrated circuits, require lesser integrated circuit chip area to provide the same number of column drivers.
It is a further object of the present invention to provide such a monolithic column driver integrated circuit wherein much of the CMOS circuitry used to generate the voltage that drives the column of the active or passive matrix LCD display is operated from a power supply range that is of a smaller magnitude than the amplitude of the column driver output signal.
A still further object of the present invention is to provide a CMOS monolithic integrated circuit that provides an output signal that can be switched through a relatively wide output range while limiting the voltage across any particular CMOS device to a voltage magnitude that is significantly less than the amplitude of the output signal.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.